Conventional surface acoustic wave (SAW) filters can provide excellent filter characteristics, but have fixed properties set in the design and fabrication. Electrically tuned filters are often needed. For example, a preselector filter may be needed in a receiver prior to amplification and/or frequency conversion, in order to suppress strong unwanted signals that could result in multiple spurious signals from mixing in nonlinear devices. Or, a filter could be needed in a frequency synthesizer to select any one of many continuous RF tones on command. Or, it could be used in the feedback loop of an adjustable oscillator to provide the very low phase noise expected from a SAW-stabilized oscillator, but with tunability over frequency without the use of the commonly used varactor, which degrades phase noise.
Electrically adjustable SAWs have been previously built and demonstrated with discrete electronic switching of individual fingers or finger groups in the SAW transducers. However, the filter properties are usually crude in terms of passband shape control and are not continuously tunable.
Thus, a need exists for passband control of a surface acoustic wave.